84.2×1.27÷4.21
=（84.2÷4.21）×1.27
=20×1.27
=25.4
3.69×31.5÷10.5
=3.69×（31.5÷10.5）
=3.69×3
=（3.7-0.01）×3
=3.7×3-0.01×3
=11.1-0.03
=11.07